This invention relates to digital demodulation of a digitally modulated analogue r.f. signal, such as is used in TDMA radio systems.
In mobile radio telephony systems, e.g. cellular systems and the like, low power time division multiplexed radio links are used between the mobile transmitter/receivers and fixed base stations. Transmission is in the form of digital bit stream burst which modulate a predefined carrier radio frequency. Various forms of modulation are known, including quadrature phase shift keying (QPSK) and Gaussian minimum shift keying (GMSK). Hence a receiver is required to demodulate an input r.f. signal which has been downconverted to form an analogue IF signal of the general form EQU y(t)=A. sin (.omega..sub.F t+.phi.(t))
when derived for a GMSK modulator. An important aspect of the overall receiver operation is to extract timing information from the incoming signal to enable satisfactory demodulation to be performed. Demodulation is achieved by sampling the waveform in each bit period and it will be understood that correct bit synchronisation is essential, particularly where the timing of the bit periods in the incoming signal is liable to slippage. Several proposals have been put forward to achieve correct bit synchronisation in a receiver. Of special interest are arrangements involving digitising of the signal followed by oversampling and digital signal processing to determine the proper bit symbol timing. Such arrangements are disclosed in U.S. Pat. Nos. 4,849,991; 4,937,841; 4,941,155 and 5,084,891.